164 



R. P. Levine 



Table III 



Plastoquinone content of wild type and 

 mutant strains of C. reinhardi 



Strain Moles chlorophyll /mole plastoquinone 



wild type 15 



ac-2j_ 30 



ac-115 99 



ac-141 "74 



DISCUSSION 



Each of the four mutant strains of C. reinhardi under consideration is 

 unable to carry out normal photosynthesis because some portion of the electron 

 transport system does not function. This loss of function results from the mu- 

 tation of either a structural or regulatory gene. The expression of this muta- 

 tion is seen experimentally as a block in photosynthesis, and the term block 

 will be used to describe the point or points at which the electron transport 

 system is stopped. It is assumed that in each mutant strain the genetic change 

 affects an enzyme that is at least indirectly concerned with the electron trans- 

 port system. The term block, however, is not meant to imply a knowledge of 

 the specific enzymes involved. Furthermore, a single mutation could result 

 in a block at more than one point. If, for example, two components of the sys- 

 tem are formed as part of the same biosynthetic pathway, a mutation that af- 

 fects some common, early step in their biosynthesis could result in the loss 

 of both components and, consequently, the system would be blocked at two 

 different points. In addition, the loss of some single component might result 

 not only in a block within the electron transport system but in the coupling of 

 photophosphorylation to the system as well. 



The initial model proposed for the electron transport system of photo- 

 synthesis in C. reinhardi<9/ was based upon the hypothesis of Hill and Bendall 

 that there are" two different, light-dependent reactions coupled by at least one 

 light-independent, exergonic reaction. According to this model, and using the 

 terminology of Duysens, light absorbed by system II results in the oxidation of 

 water coupled to the formation of a photoreductant; light absorbed by system I 

 results in the formation of a photo-oxidant and the reduction of TPN. Overall, 

 the photoreductant produced in system II is oxidized by the photo-oxidant pro- 

 duced in system I. 



The model to be discussed here (Fig. 1) retains the general features of 

 the one presented earlier^^)^ It has gained additional support from recent 

 studies of the mutant strains of C. reinhardi . As further experiments are 

 performed, and other mutant strains studied, the details of the model may 

 change. This is, therefore, only a working model, but it most nearly 



